In commercial production laser and electron beam welding applications, the goal is to maximize welding speed and minimize welding defects while attempting to weld a wide variety of steels, steel alloys, aluminum, aluminum alloys and other alloys. In fact, in most instances, welding speed is the most important limiting factor in achieving higher productivity and lowering production costs. For example, when laser welding metal sheets, the suitable welding speeds that can be achieved are typically a direct function of the penetration depth required. Hence, as sheet thickness increases welding speed typically decreases.
A commonly used strategy to increase welding speed is to increase the power density of a focused laser beam having a circular beam spot at the point where the beam is irradiating or impinging against the workpiece. As is discussed in more detail in an article in the Proceedings of LAMP '92 entitled "CO.sub.2 Laser Welding of Aluminum Alloys", and in a June 1992, International Symposium on Automotive Technology and Automation Conference Proceedings (ISATA), article entitled "ADVANCES IN LASER WELDING OF AUTOMOTIVE PARTS" increased power density can be achieved by increasing laser power or decreasing the focused spot size of the laser. Increasing laser power can obviously be achieved by using a higher power laser.
Decreasing the focused spot diameter of the circular spot can be achieved by using lasers having higher beam quality or better mode. For a laser of a given kilowatt power output, the laser having a beam quality, K, as close to unity or one as possible is preferred for providing a focused beam spot having higher power density over another laser having poorer beam quality. In addition to beam quality, K, the focused beam spot size, w, of a laser is also dependent on the laser output wavelength, .lambda., the focal length, f, of the beam focusing optic and the diameter, D, of the unfocused beam from the laser source. This relationship is more clearly illustrated by the following equation for determining the focused beam diameter, w: ##EQU1## To decrease the focused beam spot diameter, w, for increasing the power density of the focused beam at the workpiece, a laser with a smaller wavelength, .lambda., can be chosen or focusing optics with a smaller focal length, f, can be chosen. Alternatively, a laser having a higher beam quality, with K=M.sup.2 =1/k as close to the value of 1 as possible can also be chosen or a combination of the aforementioned can be selected to optimize power density of the focused beam spot.
However, as power, power density and welding speed are increased, the frequency of welding defects, weld failures and scrap rate increase to a point where welding speed can no longer be economically increased. These welding defects range from pores, pinholes, undercuts, weld sputters and, in some instances, to undesirable increases in hardness in the heat affected zone of the resultant weld. In welding metals, and particularly steel, another welding speed related defect, called humping or slubbing, can occur. Humping is discussed in more detail in an article in the Journal of Physics, D: Applied Physics 25 (1992), entitled "Theoretical approach to the humping phenomenon in welding processes". As is demonstrated in another article in the Proceedings of LAMP '87 entitled "Deep Penetration Welding with High Power CO.sub.2 Laser", as the focused spot diameter of a relatively high power CO.sub.2 laser beam decreases, power density increases, and the speed at which humping defects occur is reduced, thereby limiting welding productivity. Many of these defects are caused because the high power densities used with circular focused beam spots introduce too much energy too quickly into the region being welded.
One study of laser welding is disclosed in an article in the June 1992, ISATA Proceedings entitled "MECHANICAL PROPERTIES OF LASER WELDED ALUMINUM JOINTS", discusses attempts to reduce welding defects in certain specific aluminum alloys. This article describes training a CO.sub.2 laser beam having a power source of at least 2.5 kilowatts at an angle of incidence to the weld direction with the beam being tilted, first, in a direction perpendicular to the direction of welding and, second, along the direction of welding. When angled perpendicular to the direction of welding, the longitudinal axis of the beam spot is perpendicular to the welding direction and when the beam is angled along the direction of welding, such as by "trailing" or "leading" the beam, the beam spot has a longitudinal axis parallel to the welding direction. Unfortunately, deviations in the angle of incidence of the beam perpendicular to the weld direction up to 10.degree. to 15.degree. only slightly influence weld porosity while undesirable porosity in welded aluminum alloy increases dramatically as the angle of incidence is increased beyond about 15.degree.. In fact, porosity is minimized for both cases of angling the beam relative to the workpiece and welding direction as the beam angle of incidence approaches zero degrees and when the beam is generally perpendicular to the workpiece.
Angling the beam relative to the workpiece is also disclosed in a June 1992 ISATA Proceedings article entitled "LASER BEAM WELDING OF HSS-COMPONENTS FOR CAR BODIES". This reference discloses that in welding high strength steel (HSS), the laser beam axis can be rotated up to 45.degree. in or against the feed direction without a considerable loss in quality. Hence, beyond a 45.degree. beam angle there is a significant loss in weld quality and an undesirable increase in porosity in the weld.
It is also known that particularly for high power, typically more than one kilowatt, and high speed, typically more than 2 meters per minute, welding applications, if the angle of incidence of the axis of the beam relative to the workpiece is too small, welding defects such as porosity, pinholes, weld sputters, humping, and undercutting increase and weld quality decreases making it highly undesirable to angle the beam too close to the workpiece.
Another method of reducing welding defects for welding difficult to weld materials, such as aluminum alloys, is disclosed in the 1992 ICALEO Prooceedings in an article entitled "HIGH POWER PULSE YAG LASER WELDING OF THIN PLATE" and a corresponding publication of European Patent Appln. No. 0594210 A1, published Mar. 24, 1994, and entitled "Method and apparatus for welding material by laser beam". These references disclose using a pulsed laser and angling the beam obliquely relative to the workpiece it is irradiating to create what is referred to as an elliptically-shaped focused beam spot on the workpiece for suppressing weld cracks and pores in the weld joint of the difficult-to-weld material.
The pulsed YAG laser used in these references to weld difficult to weld aluminum alloys of thin sheet thickness has a relatively low average power of 800 watts and welds at a relatively low speed of 1.0 millimeters per second. This method is not suited for commercial laser welding applications, such as automotive sheet metal welding, tube welding, or continuous welding of roll-formed parts, which typically require a welding speed of greater than 30 millimeters per second or about 2 meters per minute to be economically viable. Additionally, low power pulse lasers of typically less than one kilowatt average power are not suited for sheet metal welding applications requiring adequate penetration, high speed and high productivity. For welding thicker sheet material, such as automotive deep drawing steel of 1 millimeters cross sectional thickness and thicker, a higher power laser is needed to meet the high industry production rates required to be economically competitive.
Additionally, these references disclose that the laser source be pulsed for initiating melt-solidification, remelt-resolidification cycles to reduce weld defects, namely pores, weld sputters and cracks. Unfortunately, pulsing requires sophisticated and expensive electronic hardware to generate and control the frequency, shape, and duration of the laser pulses. Furthermore, since the lasers disclosed are pulsed for providing a way of reducing and controlling average power density at the workpiece, the average power output of the laser is less than its peak power. Moreover, if pulsing at too high of a frequency is performed, the beam no longer initiates melt-solidification, remelt-resolidification cycles to reduce weld defects, namely pores, weld sputters and cracks.
Welding speeds that can be achieved using pulsed lasers are also limited because if the beam travels too fast along the workpiece it can leave portions of the workpiece along the weld line inadequately welded or unwelded during the periods when the beam is off or between pulses. For example, if f represents the number of pulses per second (pps) in Hertz and, d, represents cumulative or total the dwell time of a laser pulse in seconds, then, t=1/f-d, represents the time duration between pulses during which the workpiece is exposed only to a continuous component of laser power, if there is any continuous component. As a result, when welding at higher speeds with a pulsed laser having no continuous component, the workpiece is not exposed to any beam energy during the time period defined by: t=1/f-d, resulting in a section having length, l=V*t along the weld line of the workpiece that may not be adequately welded or not welded at all. If there is a continuous laser beam component and the continuous component is less than 80% of the peak power, if t is too great, sections along weld line may not be adequately welded or not welded at all. Accordingly, the larger the welding speed, V, the longer this potentially unwelded or poorly welded distance, l, between pulses becomes. If welding at high enough welding speeds, some portions of the weld joint might not be welded adequately, particularly if l&gt;0.1 millimeter. Also, if pulsing is at a frequency such that t becomes too small, the beam no longer initiates melt-solidification, remelt-resolidification cycles to reduce weld defects. This is why duty cycles of pulsed lasers are generally lower than about 50%.
Other references disclosing the use of pulsed lasers for reducing welding defects include LAMP '87, "Welding Properties with High Powered Pulsed CO.sub.2 Laser"; LAMP '87 (May 1987), "Development and Implementation of High Speed Laser Welding in the Can Making Industry"; LAMP '92 (June 1992), "High Power Pulse YAG Laser Welding of Thin Plate"; and a 1991 ICALEO article entitled "Butt Welding of Thin Stainless Steel Sheets with `Rippled Mode` Nd-YAG Laser" and U.S. Pat. No. 5,347,528.
Linear or line focusing optics have been coupled with lasers to produce a linear or generally rectangular beam spot used for surface cladding, surface hardening and surface melting workpieces as part of final finishing of these workpieces. It is not believed that heretofore these linear and generally rectangular beam spot shapes have been used to weld workpieces and more particularly to butt-weld or lap weld sheets in a commercial welding setting.